4.4 String Search and Comparison

Strings are objects, so compare their contents with methods. Do not use == to decide whether two user-facing strings have the same text.

String a = new String("java");
String b = new String("java");

System.out.println(a == b);       // false: different objects
System.out.println(a.equals(b));  // true: same text

equals(other)

equals(other) returns true when two strings contain exactly the same characters in the same order.

"Java".equals("Java");  // true
"Java".equals("java");  // false

This is the normal method for checking passwords, commands, menu choices, and exact labels.

equalsIgnoreCase(other)

equalsIgnoreCase(other) compares text while ignoring uppercase/lowercase differences.

"YES".equalsIgnoreCase("yes");  // true

This is useful when accepting commands such as yes, YES, or Yes.

compareTo(other)

compareTo(other) compares two strings in dictionary-like order.

"apple".compareTo("banana");  // negative
"cat".compareTo("cat");       // 0
"dog".compareTo("cat");       // positive

The exact positive or negative number is less important than its sign:

• Negative: the left string comes before the right string.

• Zero: the two strings are equal.

• Positive: the left string comes after the right string.

contains(part)

contains(part) checks whether a substring appears anywhere inside the string.

"error: timeout".contains("error");  // true

It returns a boolean, not the position.

startsWith(prefix) and endsWith(suffix)

startsWith(prefix) checks the beginning of a string. endsWith(suffix) checks the end.

"Report.java".endsWith(".java");    // true
"Report.java".startsWith("Test");   // false

These methods are helpful for file extensions, command prefixes, and simple validation.

indexOf(part) and lastIndexOf(part)

indexOf(part) returns the first position where a substring appears. If it does not appear, the result is -1.

String line = "error warning error";

System.out.println(line.indexOf("error"));      // 0
System.out.println(line.lastIndexOf("error"));  // 14
System.out.println(line.indexOf("debug"));      // -1

Use contains() when you only need yes/no. Use indexOf() when the position matters.